Accompanying the increasingly higher levels of integration of semiconductor devices in the semiconductor industry in recent years, there is a growing need for fine patterns that exceed the transfer limitations of conventional photolithography methods using ultraviolet light. Extreme ultraviolet (EUV) lithography is considered to be promising as an exposure technology that uses EUV light to enable the formation of such fine patterns. Here, EUV light refers to light in the wavelength band of the soft X-ray region or vacuum ultraviolet region, and more specifically, light having a wavelength of about 0.2 nm to 100 nm. Reflective masks have been proposed as transfer masks for use in this EUV lithography. Such reflective masks have a multilayer reflective film that reflects exposure light formed on a substrate, and an absorber film that absorbs exposure light is formed in a pattern shape on the multilayer reflective film.
The reflective mask is manufactured from a reflective mask blank having a substrate, a multilayer reflective film formed on the substrate, and an absorber film formed on the multilayer reflective film, by forming an absorber film pattern by photolithography and the like.
As has been described above, due to the growing demand for miniaturization in the lithography process, significant problems are being encountered in that process. One of these is the problem relating to defect information of mask blank substrates used in the lithography process.
Mask blank substrates are being required to have even higher smoothness from the viewpoints of improving defect quality accompanying the miniaturization of patterns in recent years and the optical properties required of transfer masks. Examples of conventional surface processing methods used for mask blank substrates include those described in Patent Literatures 1 to 3.
Patent Literature 1 describes a glass substrate polishing method comprising polishing the surface of a glass substrate having SiO2 as a main component thereof so that surface roughness Rms as measured with an atomic force microscope is not more than 0.15 nm using a polishing slurry, which contains colloidal silica having an average primary particle diameter of not more than 50 nm, acid and water, and is obtained by adjusting the pH to within the range of 0.5 to 4.
Patent Literature 2 describes an abrasive for a synthetic quartz glass substrate containing an inhibitory colloidal solution and an acidic amino acid for inhibiting the formation of defects detected by a highly sensitive defect detection apparatus on the surface of a synthetic quartz glass substrate.
Patent Literature 3 describes a method for controlling surface flatness of a quartz glass substrate that is capable of controlling surface flatness at the sub-nanometer level by placing a quartz glass substrate in a hydrogen radical etching apparatus and allowing hydrogen radicals to act on the quartz glass substrate.
In addition, Patent Literature 4 describes a method of manufacturing a glass substrate for an EUV mask blank, wherein irregularities are measured on the surface of the glass substrate, flatness of the glass substrate surface is controlled by carrying out local processing under processing conditions corresponding to the degree of unevenness of convex sites, and surface roughness and surface defects caused by local processing are improved and removed by carrying out non-contact polishing such as elastic emission machining (EEM) on the glass substrate surface subjected to local processing.
Conventionally, mask blank substrates have been subjected to surface processing to enhance flatness of the surface thereof by using these methods, for example.
Furthermore, Patent Literatures 5 and 6 describe the application of catalyst-referred etching (CARE) in order to flatten semiconductor substrates such as SiC, sapphire or GaN substrates.